Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material have been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
In recent years, methods to use lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a Nasicon structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known Nasicon structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high output voltage of 3.5V and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as the cathode active material for lithium secondary batteries.
Among olivine structure compounds, LiFePO4 has a high output voltage of 3.5V, a high volume density of 3.6 g/cm3, and a high theoretical capacity of 170 mAh/g, as compared to lithium (Li), and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe as an ingredient, thus being highly applicable as a cathode active material for lithium secondary batteries.
However, LiFePO4 disadvantageously causes an increase in internal resistance of batteries due to low electrical conductivity, when used as a cathode active material. For this reason, when battery circuits are closed, polarization potential increases, thus decreasing battery capacity.
In order to solve these problems, Japanese Patent Application Publication No. 2001-110414 suggests incorporation of conductive materials into olivine-type metal phosphates in order to improve conductivity.
However, LiFePO4 is commonly prepared using Li2CO3 or LiOH as a lithium source, by solid state methods, hydrothermal methods and the like. Lithium sources and carbon sources added to improve conductivity disadvantageously cause a great amount of Li2CO3. Such Li2CO3 is degraded during charging, or reacts with an electrolyte solution to produce CO2 gas, thus disadvantageously causing production of a great amount of gas during storage or cycles. As a result, disadvantageously, swelling of batteries is generated and high-temperature stability is deteriorated.
In addition, the related patents by the H.Q company disclose physically coating carbon on LiFePO4 However, when surface coating is simply performed through physical bonding, uniform coating is impossible, as can be seen from the following Test Example.
Specifically, since oxygen is present on the surface of olivine-structure particles, and oxygen and carbon cannot be present on the surface though chemical bonding, if they are chemically bonded to each other, they are converted into CO or CO2 gas and cannot be present on the surface of particles. Accordingly, when carbon coating is performed on the olivine particle surface in a simple physical manner, bonding force is considerably weak and the coating may be readily separated even by a slight impact. In particular, in the process of mixing an electrode slurry, active materials and carbon are separated, and, as a result, the results obtained when excess conductive material is added to the electrode occur. This may cause a deterioration in electrode density.
Accordingly, in order to improve conductivity of LiFePO4, there is an increasing need for techniques in which carbon coating is uniform and active materials and carbon are not separated from each other during slurry mixing.